1. Field of the Invention
The present invention generally relates to a method of manufacturing a steel member such as, for example, automobile structural component parts, and more particularly to the method of manufacturing the steel member of high physical strength and toughness having a bainitic structure as a principal structure.
2. Description of the Prior Art
Hitherto, a steel member which requires a tensile strength within the range of about 70 to 100 kgf/mm.sup.2 (corresponding to Vickers hardness Hv of 220 to 320) has often been manufactured by heat-treating (tempering) carbon steel or alloyed steel.
Recently, however, a non-tempered steel having a physical strength comparable to the tempered steel has become widely used. This non-tempered steel is, for the purpose of reducing the cost incurred during the heat-treatment, manufactured by hot-forging the material added with vanadium and subsequently air-cooling it. The non-tempered steel so manufactured by the process of hot-forging followed by air-cooling tends to result in a coarse grain size in crystalline structure and exhibits a substantially pearlite structure. Therefore, as compared with the tempered steel, the non-tempered steel low in toughness (that is, low in impact strength) and, accordingly, the application thereof is limited.
On the other hand, the Japanese Patent Publication No. 61-31184, published July 18, 1986, discloses a technique of manufacturing a steel product having an excellent resistance to any one of fatigue, wear, pitting and spalling. As the starting material, the method of the above described publication employs the non-tempered steel which is of a particular composition containing carbon (C) in a quantity within the range of 0.15 to 0.35 wt. %, silicon (Si) in a quantity of not greater than 0.35 wt %, manganese (Mn) in a quantity within the range of 0.60 to 1.30 wt. %, chromium (Cr) in a quantity within the range of 0.70 to 1.50 wt. %, vanadium (V) in a quantity within the range of 0.05 to 0.50 wt. %, nitrogen (N) within the range of 0.006 to 0.020 wt. %, and solid aluminum (sol. Al) in a quantity within the range of 0.02 to 0.10 wt. %, the balance being iron (Fe) and indispensable impurities, relative to the total weight of the non-tempered steel. The publication also describes that one or more of sulfur (S) in a quantity within the range of 0.04 to 0.13 wt. %, lead (Pb) in a quantity within the range of 0.03 to 0.35 wt. % and calcium (Ca) in a quantity within the range of 0.0010 to 0.0100 wt. % may be employed in the composition of the non-tempered steel.
According to the above mentioned publication, the starting material, that is, the non-tempered steel is first heat-treated at a temperature higher than the A.sub.3 transformation point and is subsequently cooled. In order to obtain a structure including ferrite and pearlite in a surface region of the non-tempered steel so that the subsequent nitriding processing can develop extreme hardness in the surface region of the non-tempered steel, the cooling of the heat-treated steel from 800.degree. C. down to 500.degree. C. is carried out in a controlled fashion, specifically at a cooling speed of 2.degree. C. per second or lower.
The reason that the non-tempered steel generally has a low toughness is partly because the steel is made up of pearlite or a combination of ferrite and pearlite and partly because the crystalline grain size is coarse.
As regards the reduction in toughness as a result of the presence of the pearlite or a combination of ferrite and pearlite, it is well known that the toughness of the steel may be increased if the steel is made up of bainite or a combination of ferrite and bainite. While isothermal cooling from the austenitizing temperature (about 730.degree. C.), that is, austempering, would be effective to develop the bainite structure, the austempering process lacks a practical applicability due to processing time and cost.
Therefore, to develop the bainite structure with the use of a continuous cooling, or anisothermal cooling technique, an attempt have been made to add one or both of Mn (manganese) and Cr (chromium), which are an element used to improve the hardenability, or to adjust the amount of one or both of Mn and Cr used. However, this attempt makes it difficult to determine the appropriate cooling speed and hence to form the stable bainite structure. In other words, if the cooling speed is excessively low, the steel will become a pearlitic structure, but if it is excessively high, it will become a martensitic structure.
A reduction in toughness of the non-tempered steel as a result of the coarse crystalline grain size may be avoided if the steel material is, after the hot-forging, normalized to make the grain size to be greater than No. 6 (rating according to Japanese Industrial Standards, JIS, or ASTM). However, the finer the grain size, the more is the hardenability of the steel lowered, and it will become difficult to form the stable bainite or a combination of bainite and ferrite in the ordinary steel.
As hereinabove discussed, since the amount of the hardenability improving element or elements used, the cooling speed and the grain size and closely related with each other, no technique has yet been available to form the stable bainite or a combined ferrite and bainite structure by the use of the continuous cooling.